In the 1960s, when the rival West Coast Main Line was electrified, express services from London to Birmingham on this route were discontinued as part of the Beeching Axe; beforehand, it was even more heavily used by many long trains running from Liverpool and Birkenhead, as the WCML was restricted in capacity due to the electrification works. All local trains on the route were diverted to Marylebone in 1963, and Greenford station on the New North route between Old Oak Common and Northolt Junction was run down and eventually closed. The route was downgraded to secondary status in 1967, and subsequently single-tracked between Princes Risborough and Aynho Junction, which remained a flying junction. Snow Hill station in Birmingham was also closed, along with the line to Wolverhampton.

The route was eventually considered for partial closure in the 1980s, with all services returning to Paddington via the New North route, and Marylebone station and all lines leading to it being closed - services to and from Aylesbury would have run via Princes Risborough. Marylebone was formally reprieved in 1986, however, and the closure proposals were rescinded.

Services were expanded somewhat in the late 1980s, when Snow Hill station was re-opened, although they still ran from Marylebone rather than Paddington. In the early 1990s, the New North route between Old Oak Common and Northolt Junction was singled between Old Oak Common and Park Royal and also between Greenford and Northolt Junction. The Total Route Modernisation performed by BR in the early 1990s removed most of the vestigial traces of main line heritage from the route, downgrading it purely to a commuter line with a minimum of available infrastructure; until that point, High Wycombe station alone had retained almost all of its original track layout, the other major stations on the line having already been downgraded. In 1992, the old signal box at Aynho Junction was closed and replaced with modern signalling controlled from Banbury South signal box; the structure stood until 2002, when it was demolished. As part of these renovations, BR also installed the advanced Automatic Train Protection (ATP) system mainly as a trial with a view to rolling it out nationwide. However, privatisation intervened, and the Great Western Main Line was the only other line to be equipped with ATP.

As of September 2006, Chiltern has completed their Evergreen 2 upgrade project. The project, which was carried out by Carillion, realigned the track through Beaconsfield to increase non-stop speeds from 40mph to 75mph, installed additional signals between High Wycombe and Bicester North (as well as between Princes Risborough and Aylesbury), and added two new platforms to Marylebone. The new platforms are on the site of the old daytime carriage sidings, which were replaced with the new Wembley Light Maintenance Depot, just to the south and west of Wembley Stadium station. The new platforms and partial resignalling of the station throat now make it possible to run 20 trains per hour in and out of the station.

Future

All services to and from Birmingham Snow Hill could be operated by Chiltern Railways.

A new station could be built in Aylesbury, called Aylesbury Vale Parkway. The planned site is 2 miles along the Calvert freight-only line, which would have upgraded signalling fitted. The service would be hourly, with up to 3 arrivals / departures per hour in the peak times. The station is expected to open in 2009.

Building of the West Hampstead Interchange to allow easy interchange with the Silverlink Metro, Jubilee line, Metropolitan Line and Thameslink line. This would give Chiltern Railways an interchange with the future Orbirail line.

A new railway built between Oxford and Princes Risborough, this would then give Oxford an alternative to the Oxford-Paddington route. The Oxford to Banbury spur would then be handed over to the Chiltern mainline to create a diversionary loop from Princes Risborough to Banbury via Oxford. This option requires an expensive crossing of the M40 motorway.

Plans that are at an early stage have proposed a Crossrail type scheme to link Marylebone and Fenchurch Street station. This would give extra east west capacity through London and link two of the smaller rail franchises that could then be amalgamated.